The tiny freshwater zebrafish, a favourite in aquariums, has now helped researchers understand the neural mechanism involved in fast movement. The fish needs to swim to battle the drift of the streams and the question of how they do this intrigued the researchers from the National Centre for Biological Sciences (NCBS), Bengaluru.
The team, consisting of Urvashi Jha and Vatsala Thirumalai studied a reflexive behaviour called optomotor response in freely swimming zebrafish larvae. They were able to pinpoint how speed was controlled during this behaviour at the level of single nerve cells.
The team evoked the optomotor response in the laboratory by moving black and white bars on a little screen placed under the fish. When fish were placed in normal water, they swam to keep up with the moving bars. However, when drugs that activate dopamine receptors were added to the water, the fish swam much faster and even got ahead of the moving bars. They noticed that the fish swam faster by bending the tail more from side to side. The results were published in Current Biology.
Tail movement is caused by motor neurons sending electrical impulses to the muscle. By recording the electrical activity of single motor neurons, they showed that the increased tail bending was due to dopamine’s direct actions on motor neurons. These results are exciting because they show that motor neurons, which are thought to mostly only relay the command coming to them, are capable of altering the behavioural output.
Ideal model organism
Ms. Jha, the first author of the paper, explains that zebrafish served an ideal model organism as it allowed in-depth studies from the level of behaviour to single neuronal levels. “We have currently studied only the motor neurons, and it would be interesting to see if dopamine affects the properties of other neurons in the spinal cord too. Another follow up study can be to investigate what exact information dopamine is encoding,” she adds.
“We have shown that there is a lot of plasticity within the spinal cord. We now know that even after the brain has issued the command for a movement, changing the properties of motor neurons can alter the final behavioural outcome,” says Prof. Thirumalai.